How to Build an Arched Stone Bridge
DEAR TIM: I was traveling on a fall foliage road trip in southwest New Hampshire and saw a gorgeous double-arch bridge. The historical marker sign said that no mortar was used in the construction. It was so stunning I want to construct a much smaller version in my yard. Is this a crazy idea and even possible? What are the steps I need to take to get started? What's the biggest mistake that will lead to the collapse of the bridge? How much weight do you think it could support? Ann L., North Bend, OH
DEAR ANN: You may think I'm nuts, but I believe I know the exact bridge you saw! It's on Route 9 near Hillsborough, NH. There are several of these dry-laid stone arch bridges in this part of New Hampshire, most built between 1835 and the 1850's. These bridges rival the stone arched aqueducts built by the Romans. Some of these stone arches have survived for nearly 2,000 years.
The short answer is you can build a small version of a dry-laid stone arch bridge. To achieve success, you must take your time and cut the stones so they fit tightly against one another. You need to use very strong stone that will not crumble. Granite is such a stone and was used for the bridge you saw on your trip. Fine-grained granite is the easiest to work with as you can get it to split creating a quite smooth face.
Because the stone arch is three-dimensional, the stones you cut will end up being trapezoidal in shape when you look at the sides. They'll not be simple rectangles like a normal brick. You can determine the actual shape and angles of the trapezoids by making a scale drawing on a large sheet of cardboard. All you have to do is draw the concentric arcs of the underside and topside of the bridge on the cardboard and then draw in the lines that represent the joints between each of the stones.
You'll have a much easier time if you employ cement mortar as part of the project. Don't underestimate the great skills the master stone masons employed to build the bridge you saw. I doubt you'll be able to conjure up this magic, but perhaps your drive and determination will prove me wrong.
Before you balk at using cement mortar, understand you don't need that much to do the job. What's more, on the sides of the bridge that you and others will see, you can cut the stones tighter so it looks like the bridge was indeed dry laid with no mortar. I feel that's a good compromise.
The use of mortar allows you to not worry about how tightly the stones must fit. The mortar insures solid contact between the stones so there's no movement. If the stones in an arched bridge move or shift when they're loaded from the top with traffic, there's a strong possibility the bridge can collapse. Have I convinced you to use mortar? Good!
The biggest reason stone arches fail is the lower edges of the arch kick out when the top of the bridge is loaded with weight. It's imperative the base stones of the bridge are set into bedrock or a solid foundation that will resist this sideways movement. The foundation must resist the weight of the stones used in the bridge as well as any weight added to the top of the bridge once it's complete.
You can pour concrete to create this foundation below the surface of the ground. Placing this foundation below grade ensures you'll only see stone once the bridge is complete. The top of the concrete should be slanted to accept the first stones used in the arch. To determine this angle, I suggest you build a tiny model of your bridge or make a detailed scale drawing. My guess is you'll discover a 35-degree angle is perfect.
You'll need to build a wood form that supports the stone as you build the bridge. You can use thin plywood and plenty of framing lumber to support the weight. This form, once in place, must be solid and not move as you lay stone after stone. A week after the stone bridge is complete, you can remove this form.
It's imperative you construct the bridge equally from each side working towards the center. This method evens out the load on the wood form and the first row of stone of the arch. If you do decide to do a dry-laid arch with no mortar, this is very important.
As you build the arch, the ends of the bridge will have more material stacked on top of each other than the center of the bridge. This weight and mass helps put the stones in compression which works to hold the bridge together. It's important these stones be laid tightly so there is no movement. Once again, add the weight equally on each side of the bridge as you build it.
If you want to test your methodology, I suggest building a very small scale bridge using wood. You can cut the pieces of wood easily with a saw to make them fit tight. You'll discover that you don't need glue to build a very strong arch bridge. The key is to make sure the bottom of the arch can't kick sideways. When you see your model wood bridge function properly, then you can start your real stone bridge! Your finished stone arch bridge will support your weight and that of a pony for sure. Good luck!